A gas turbine, in particular a non-stationary gas turbine, usually has an operating life of about 30 years. During this period of time, technical innovations, particularly for improving the thermal efficiency of the gas turbine, have hitherto become commercially viable. It is desirable that an already installed gas turbine is equipped at least partially with these technical innovations, in order thereby to achieve a corresponding increase in efficiency in the gas turbine. To implement the technical innovations in the already installed gas turbine, it is customary to provide modified components which have the technical innovations and are exchanged with their older counterparts in the gas turbine. The aim is to exchange as small a number of components as possible in the gas turbine in order to keep the outlay in terms of upgrading and the costs of this as low as possible.
The stationary gas turbine for power station use is conventionally designed in an axial type of construction. The compressor of the gas turbine usually has a plurality of stages arranged axially one behind the other in the throughflow direction, each stage comprising moving blades assigned to a rotor and forming a moving blade ring and guide blades forming a guide blade ring. The moving blades rotate, together with the rotor, with respect to the fixed guide blades, while, in particular, the fluidic efficiencies of the moving blades and guide blades determine the fluidic efficiency of the stage. It has been possible in recent years to achieve considerable improvements in stage efficiency, using numerical flow solvers in the profiling of the blades. Thus, it is desirable that, particularly for the compressor of the gas turbine, the existing blading is removed and is replaced with blading designed from modern standpoints. The rotor remains in the gas turbine, and therefore the mechanical interface between the blading and the rotor at the blade fastening remains unchanged.
If, however, the profiling of the new blading leads to a change in the centers of gravity of the airfoils, the mechanical stress upon the blade fastening changes. As a rule, this leads to adverse loads upon the blade fastening, therefore the profiling of the new blading has to be coordinated with the maximum permissible load values of the blade fastening. As a rule, this results in restrictions in the profiling of the new blading, the result of this being that efficiency increases which can be achieved in principle cannot be implemented.
Particularly where the compressor rotor blades are concerned, it became clear that, in profiling, introducing a rearward sweep in the lower half of the flow duct increases the aerodynamic performance of the compressor rotor blades. This, however, entails a corresponding displacement of the blades' centers of gravity and of the blade trailing edge in the direction of the downstream guide blade roll. Owing to this displacement of centers of gravity, the recently prevailing symmetry of the load upon the blade fastening within the blade grooves is impaired, so that the degree of rearward sweep is severely limited in the profile of the compressor rotor blades. In order nevertheless to provide the compressor rotor blades with the rearward sweep, the deviation of the stringing line of axial sections of the new blading compared with the stringing line of axial sections of the old blading was limited within a still permissible range. What was achieved thereby was that the load in the blade root and at the circumferential groove does not become inadmissibly high when the gas turbine is in operation. On account of this, however, an appreciable potential and efficiency improvement remained unused.
This could be remedied by shortening the chord length of the blade root of the new blading. However, this measure would be disadvantageous in terms of the aerodynamic load bearing capacity and stability of the new blading and its structural integrity.
US 2003/012645 A1 describes a method for retrofitting a guide blade, in which an existing guide blade is replaced by a new guide blade.
WO 2009/103528 A2 discloses compressor blades having a sinusoidal sweep. The sinusoidal form of the sweep results in transition sweeps. This is intended to satisfy requirements with respect to increased efficiency and noise control